A minimalist Drupal 7 module that adds a header to every email sent from a Drupal site. This module creates a callback used by compatible email providers (such as Gmail or Outlook), which lets the email recipient click on an "unsubscribe" link instead of reporting your email as spam.
Just having this feature may get your emails out of your users' spam folders, depending on their provider.
For more infos, see here => http://www.list-unsubscribe.com/
A similar module exists (https://drupal.org/project/unsubscribe) but it does not add the header nor lets you add non-registered emails (i.e. emails which belongs to no registered user).